The present invention relates to a method for manufacturing a thick oxide layer on a semiconductor substrate using a trenching method.
As is well known, a process for manufacturing a layer of thick oxide on a portion of semiconductive substrate of a wafer comprises the following steps: the formation of a thin layer of oxide; the formation of a layer of nitride, for protecting the portion of semiconductive substrate where the thick layer of oxide will not be formed; and the carrying out of a thermal treatment in wet oxidizing atmosphere (02 wet).
According to the prior art, in order to obtain a layer of oxide, for example, of 3 xcexcm it is necessary to carry out a thermal treatment in a wet oxidizing atmosphere of the wafer for about 15 hours at a temperature of 1100xc2x0 C. This is an example of a LOCOS (local oxidation of silicon) oxide process. Some of the problems with the LOCOS process are: formation of a xe2x80x9cbird beakxe2x80x9d structure, a lack of planarization of the structure, the requirement of a very high temperature, and a long oxidation time. Because of these problems, oxide layers are generally not grown more than 2-3 xcexcm thick.
FIG. 1 shows a portion of semiconductive substrate 1xe2x80x2 of wafer on which a thick layer of oxide 4xe2x80x2 has been formed according to the prior art. As is evident from the drawing, after having carried out the thermal treatment, the layer of oxide 4xe2x80x2 is also formed between the layer of nitride 3xe2x80x2 and the substrate 1xe2x80x2 connecting with the thin layer of oxide 2xe2x80x2. In particular, the layer of oxide 4xe2x80x2 extends itself under the layer of nitride 3xe2x80x2 by an amount which is substantially about its thickness Dxe2x80x2. This is the bird beak structure.
Furthermore, the surface of this layer of oxide 4xe2x80x2 is raised, with respect to the surface of the substrate 1, of an amount equal to half the thickness Dxe2x80x2 of the layer itself.
Therefore, after the formation of the layer of oxide 4xe2x80x2, the entire surface of the wafer is not planar.
A known technical solution for planarizing the surface of the wafer is described in the book: xe2x80x9cSILICON PROCESSING FOR THE VLSI ERA,xe2x80x9d VOL. II, page 42, hereby incorporated by reference.
Thermal treatment with high temperatures is also necessary in order to carry out this process of planarization.
Embodiments of the present invention are directed toward a method for manufacturing a thick layer of oxide, having such characteristics which allow the formation and subsequent planarization with a low thermal temperature that overcomes the limitations and/or the drawbacks which, at the moment limit the methods for forming thick layers of oxide according to the prior art.
Embodiments of the invention form, in the portion of substrate where the formation of a layer of thick oxide is desired, a plurality of trenches, before carrying out conventional thermal treatments.
Characteristics and advantages of the invention will be seen from the description, following herein, of an embodiment given as an indication and not limiting with reference to the drawings attached.